The mouse t complex contains several genes involved in sperm development. Naturally occurring mutations of these genes exist in a variant form of the t complex called the t haplotype. The phenotypes of these mutations include sterility, reduced fertility, and transmission ratio distortion (TRD). This proposal seeks funding to continue investigations into the genes responsible for TRD. The primary aim of this project is to characterize the function of the t complex responder gene (Tcr) in t haplotypes. Tcr is crucial for the process of TRD, a mechanism in which nearly all offspring sired by a male heterozygous for a t haplotype (+/t) inherit the t-bearing chromosome. From the population genetics and evolutionary viewpoint, this is a classic example of meiotic drive. TRD involves at least three t haplotype-encoded "distorter" loci acting in concert with Tcr to cause distortion (in +/t males) and sterility (in t/t males). Genetic evidence indicates that the genes involved in TRD and sterility are the same. Therefore, an understanding of TRD should directly apply to the mutations affecting male fertility. Tcr is the only mammalian gene known that causes an individual to produce functionally inequivalent male gametes. To investigate TRD, homologous recombination in embryonic stem (ES) cells will be used to mutate a putative Tcr gene. Mutant mice derived from the altered ES cells will be bred to determine if the lesion obliterates TRD. A separate targeting experiment will yield mice deficient in wild-type Tcp-10 function, allowing an evaluation of the "normal" function of this gene in spermatogenesis. Breeding analysis will reveal whether products of the candidate gene (Tcp-10b/t) and related homologs have the ability to diffuse across the spermatogenic intercellular bridges, a process whereby genetically non-identical gametes can be functionally equilibrated. The long range goal of this work is to understand the unique developmental events leading to sperm differentiation in mammals, and how independent genes can rapidly co-evolve mutually beneficial activities (TRD of t haplotypes). The transmission ratio distortion system, in which several interacting genes have been identified, is a unique and powerful tool to dissect these facets of mammalian development and genome evolution.